Push Button in FBL5N for Dispute case creation

Hi Experts,

I am new to Dispute Management and i have been trying to create dispute case for a customer line item. The button which is displayed in FBL5N report is greyed out because of which i am not able to create dispute case.

Please help me in solving the problem....how should i create a dispute case for a customer line item.

Regards

Abhi.